[altvnk]

Diagnostic-only scope

Actual-call observations for unchanged review_cases.json#72 (rv_next_clipped_step), limited to five owned runner/FFI/shim/support paths. No reconstruction or production/reference changes. Original fixtures, generator, 189-value serialization/comparisons, known-mismatch exception, guards, 1e-9 relative tolerance, empty absolute floors and unit plan status are preserved. This is not a numerical fix or unit promotion.

Current verified head

• Tested head: 6514e96a3c2b47a7e577e8d5541618f4731a1109.
• Clean attributed merge of authoritative main at 27b698e905f033b4a6a96548fbc65b2bfae04294; final freshness check confirmed that same tip.
• Canonical CI 39972227711, attempt 1: all seven jobs passed, including native Clippy and runner tests. No local builds/native tests or retries.
• Native test job 120477606159: both per-side original 189-value records remain bit-identical; checkpoint test passed; runner known-mismatch controls passed.

Four actual observations

Tag / length	Observed boundary	Measured result
1 / 12	Plane input	All values bit-identical.
2 / 82	Plane output	First observed difference: component 8, point.uv_curve_pv[0][0].y: C++ -0.09999999999999992, Rust -0.1 (six ULPs; below 1e-9 relative). Other intermediate matrix/position fields also differ.
3 / 81	Returned step/point	Rescaled step bit-identical; UV and intermediate-field differences persist.
4 / 81	ComputePointValues output	Three tiny differences, all below 1e-9 relative; additional intermediate matrix/position differences are no longer present.

Root cause remains unproved. These observations do not establish causality for the final deviation mismatch. Internal plane-solve operations, LocalCurveIntersect, Hermite and span calculations remain unobserved; intermediate/inactive fields are not automatic root-cause evidence.

Unchanged native corpus

Native difftest job 120477606398:

PARTIAL blend.fillet_intersector cases=1292 passed=1291 failed=0 deferred=0 known_mismatch=1
known-mismatch rv_next_clipped_step: values.rec[3]: cpp=3.331303480692949e-16 rust=3.3306690738754696e-16
difftest: 260 unit(s), 189 passed, 71 partial, 0 failed

The original strict-gate mismatch and exception remain unchanged; the unit stays partial. Any numerical fix requires separate causal evidence and approval.